One or more embodiments of the present invention relate generally to the field of steam turbines. In particular, embodiments of the present invention relate to the servicing of such turbines, such as the disassembly, reassembly, and alignment of components as a turbine is serviced.
Steam turbines are typically constructed using one or more removable upper portions (e.g., upper shells or casings) to allow access to components within the generator itself. The components within the turbine may include a large number of stationary and rotating components. Rotating components may include one or more wheels, shafts, bearings, etc. that rotate during the operation of the turbine. Stationary components may include one or more stationary wheels, diaphragms, support pads, deflectors, etc. that remain stationary during operation of the turbine. Turbines may also include one or more lower portions (e.g., lower shells or casings) that generally serve as a support for the other turbine components, and may also assist in sealing the steam path to prevent leakage.
Close tolerances among the various components of a turbine directly affect its efficiency. To illustrate, a large steam turbine weighing several tons may have tolerances for internal components measured in millimeters (mm), or in thousandths of an inch (mils). If stationary and rotating components are too close to one another, rubbing between the components may occur during operation. This rubbing makes it difficult to start the turbine after a servicing or overhaul, and generates excessive vibration. The rubbing also wears away the seals between the rotating and stationary components, and after the components have worn themselves free, excessive clearance will then exist in the areas in which rubbing occurred.
If stationary and rotating components are too far apart from another, steam leakage may occur between the components, reducing the efficiency of the turbine. Accordingly, great care is desirable when servicing or maintaining a turbine to ensure that the various components are aligned and positioned correctly.
During an offline servicing or overhaul of a turbine, various components of a turbine may be accessed by removing the upper casing or casings, commonly referred to as xe2x80x9ctopsxe2x80x9d. With the tops off, stationary and rotating components of the turbine may be inspected, adjusted, cleaned, repaired, replaced, and/or otherwise serviced. One type of inspection may determine the amount of displacement suffered by various components due to turbine operation. For example, certain stationary components might have shifted in alignment. Components that have become misaligned may then be realigned as a part of this inspection. Upon completion of the servicing or overhaul, the upper casings may be replaced, and the turbine returned to operation.
Unfortunately, an alignment problem commonly occurs when the tops are placed back on the turbine. The upper casings may weigh one ton or more, and the placement of these upper casings onto the turbine may cause an additional amount of displacement or distortion among the previously-aligned components. Such displacement may generally be referred to herein as tops on displacement. For example, a lower casing might bow or sag between support points when in the tops off condition, and one or more stationary components connected to the lower casing may shift as well. If the components are aligned with the tops off, they may shift when the tops are placed back on, and may actually shift out of alignment.
To address this problem, it is conventional to conduct a tops on/tops off alignment procedure. In this procedure, the upper casings are first removed and the various components are serviced and/or aligned as needed. After these components are serviced, the upper casings are replaced, and the various components are measured for position both vertically and transversely with respect to the centerline of the unit. Then, the upper casings are once again removed, and a tops off line is measured. The tops off line measures the transverse and vertical positions of the internal components with the upper casings and/or components removed. Then, these measurements are compared to determine an ideal line for the internal components when in the tops off condition.
Then, with the upper casings removed, the components are then adjusted to account for the tops on displacement. When the tops are placed back on, the components are then expected to shift into alignment. For example, a set of tops on and tops off measurements might show that a particular component shifts upwards 10 mils when the tops are placed on. This component may be aligned, in the tops off condition, to be 10 mils low to account for this rise.
The tops on/tops off procedure described above helps to ensure that various turbine components are in optimal alignment at the completion of the servicing. However, the tops on/tops off procedure is time consuming. Many hours are required to perform the various measurements, as well as removing and replacing the upper casings, resulting in higher costs for personnel time and a greater amount of lost revenue due to the turbine being offline. Consequently, there is a need for a turbine servicing procedure that can reduce the time needed to align various components of a turbine.
One or more embodiments of the present invention help to reduce the time needed to align turbine components, and will be summarized below. Briefly, in one embodiment, a series of measurements are taken to determine a relative displacement exhibited by various components in their tops off state. With these measurements, a predictive algorithm is employed to determine one or more predicted offset values to compensate for the tops on displacement. The various components may then be adjusted, while the turbine remains in a tops off state, to account for the anticipated tops on displacement. When the tops are then replaced, the components may displace themselves into alignment, reducing the need for a separate series of tops on measurement, removal of tops, and realignment of components.
In a further embodiment, a topographical distortion map may be generated using a reference plane through one or more support points located on a lower casing or shell. From this plane, the vertical displacements at one or more points in the turbine may be measured, and from these displacements, predictions of vertical and/or transverse displacements may be calculated. These predictions may then be used to adjust one or more components in the tops off condition to account for displacement occurring when going to the tops on condition.
These and other embodiments and features are described in greater detail with reference to the attached drawings and the descriptions appearing below.